Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products and especially, but not exclusively, portable products such as mobile phones, audio players, video players, personal digital assistants (PDAs), various wearable devices, mobile computing platforms such as laptop computers or tablets and/or games devices. Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality, lower power consumption and reduced cost.
Micro-electromechanical-system (MEMS) transducers are finding application in many of these devices. These may be, for example, capacitive transducers for detecting and/or generating pressure/sound waves or transducers for detecting acceleration. MEMS capacitive microphones typically comprise a first electrode, which is moveable with respect to a second fixed electrode in response to incident acoustic waves. The first electrode may, for example, be supported by a flexible membrane. By measuring changes in the capacitance between the electrodes the incident acoustic signals can be detected. In use the electrodes of the MEMS microphone may be biased by biasing circuitry and the measurement signal may be amplified by amplifier circuitry such as a low-noise amplifier.
FIG. 1 shows an example of a typical arrangement of a capacitive transducer apparatus 100. In use a biasing voltage VB is applied to one plate or electrode of the capacitive sensor 101, e.g. a MEMS microphone. The bias voltage is typically generated by a voltage bias generator, which may typically comprise a voltage regulator or DC-DC converter such a charge pump 102 which receives a supply voltage VDD and generates the bias voltage VB. The bias voltage may be applied to the capacitive transducer 101 via a bias filter comprising bias impedance 103 and bias filter capacitance 104. The bias impedance 103 is arranged in the biasing path between the charge pump 102 and capacitive transducer 101 and the bias filter capacitor is connected between a node of this bias path and a controlled voltage, which in this example is ground.
Another electrode of capacitive transducer 101 is connected to a defined voltage, in this example, also ground, via a high impedance 105. This provides a signal voltage VIN resulting from deflections of one electrode of the capacitive transducer 101 with respect to the other. This signal voltage VIN is amplified by an amplifier arrangement 106 to provide an output signal SOUT.
Typically the biasing and readout circuitry are integrated together as part of an integrated circuit on a semiconductor die 107, i.e. as part of the same chip, which thus has at least die contacts 108a-c, e.g. die circuit pads or pins, for receiving VDD, connecting to ground and outputting the output signal SOUT respectively. The die 107 is typically mounted on an apparatus substrate, such as a PCB (printed circuit board) substrate (not shown in FIG. 1) with connections made between the die 107 and the PCB substrate and then the die 107 and PCB substrate are packaged in some suitable package. The package will have various external contacts, e.g. pads or pins, that are electrically connected to the circuitry within the package to allow for suitable electrical connections to be made to the package, e.g. for supply voltage, ground and an input/output signal. In a host device the packaged microphone may be mounted on some circuit board or other device substrate.
In some implementations the capacitive transducer 101 may be co-integrated with the biasing and readout circuit as part of the same die or chip 107, but in other instances the capacitive transducer may be formed on a separate die 107a which is packaged with die 107 and appropriate circuit connections made between the two dies.
There is a continual drive to reduce the size and cost of such MEMS microphones and to minimize the area and space taken up with such transducers, for example to enable the use of MEMS transducers that are suitable for embedding in earbuds for noise cancellation or other requirements for acousto-electrical feedback such as speaker linearization.
Increasingly however there is also a desire for improved performance and consistency from such transducers. Two issues that may impact performance are part-to-part manufacturing variations, that may result in different examples of the same transducer product exhibiting different sensitivity to a given input stimulus, and electromagnetic interference, that may result in unwanted noise.